Heat oven to 180°C (170°C for dark or nonstick pans). Grease two 23 or 20cm round cake pans.
Make and bake cake as directed on box for 20 or 23cm round cake pans. Cool in pans 10 minutes. Remove from pans to cooling racks. Cool completely, about 30 minutes.
Meanwhile, in a bowl, microwave chocolate chips and 120ml of the whipping cream uncovered on High 45 to 60 seconds; stir until smooth and melted. Refrigerate 15 to 30 minutes or until cool.
In large bowl, beat remaining whipping cream and the powdered sugar with electric mixer on high speed until mixture starts to thicken. Add melted chocolate. Beat until stiff peaks form (do not overbeat or mixture will begin to look curdled).
On a serving plate, place 1 cake layer, rounded side down. Spread raspberry jam over cake layer. Spread 1-cm-thick layer of chocolate mixture over jam. Cut 75 grams of the raspberries in half; press into chocolate layer. Top with other cake layer, rounded side up; press lightly. Frost side and top of cake with remaining chocolate mixture. Refrigerate about 1 hour or until firm. Let stand at room temperature about 10 minutes before serving. Garnish with remaining raspberries. Sprinkle with cocoa. Store loosely covered in refrigerator.